Religions are human creations, not god creations. Before modern religions took over humanity, humans were not considering "purpose", they were living, fucking, surviving, eating. Purpose is a deeply religious concept, man-made.

Theism can be regarded as a form of radical skepticism. Yes, that’s right – radical skepticism. The theist denies that knowledge is possible. The theist would argue that no amount of experimental investigation, data gathering, mathematical derivation or intuitive insights can ever possibly produce true answers. So the only recourse is to blind faith. By this “logic”, if I conceive of a purpose for my life, that conception is either deluded or irrelevant. If it is contrary to god’s purpose for me, than I’m deluded. If it agrees with god’s purpose, then my further delineation of purpose is irrelevant.

We have two possible solutions to this conundrum. The first is to assert that genuine knowledge really is possible, and that humanly tractable means for knowledge-attainment do exist. Then we must prove this assertion. This at least allows us to claim this or that purpose for our own lives. But such a proof is difficult because it risks circular reasoning: how to avoid assuming that any knowledge is possible, en route to actually proving that some knowledge is possible?

The alternative is to deny that faith-based knowledge is any more possible than intuitive or scientific knowledge, for we can be just as deluded in our faith as in our reasoning. The theist might be passionately committed to his religion, but his commitment is no more perfect than his knowledge. And even if he is “born again” through supra-physical transformation, his physical self has no data on which to assert that such transformation has actually occurred. Faith, in other words, offers no solution at all.

But back to the question of purpose. My personal purpose is to attempt to elucidate my purpose. I have not yet obtained sufficient knowledge on the basis of which to identify a purpose. To be charitable, kind, deferential and reasonable are all laudable attributes and good guides for personal behavior, but they are not a “purpose”. Fulfillment of duty, such as between parent and child, is certainly necessary and important. But the mere doing of one’s duty, however assiduously and successfully, is a valid purpose only for robots. We might say: “I gain deep pleasure and fulfillment from doing my duty, whereupon it’s no mere duty, but a high privilege and an ennoblement”. This is not to be gainsaid. Finding a vocation where we are both effective and fulfilled, is a wonderful thing, and a very worthy lifetime achievement indeed. Persons attaining it are to be commended. But again, is this really a “purpose”? After all, success itself, as an abstract notion, is not a purpose either.

I am tempted to opine that the very question of “purpose” is ill-posed. Perhaps true purpose is impossible for a self-aware being, for self-awareness negates the liberating imperative of mere duty. Religion, then, is an attempt to regain such imperative. The ultimate delusion isn’t that god exists, but that the postulated existence of god supplies us with a ready-made purpose.

You’re quite right about the danger of slippery phrases such as “genuine knowledge”. Radical skeptics have a leg on which to stand, precisely because of the charge that talking about knowledge itself requires knowledge, and we rapidly slip into circular reasoning.

For me, “genuine knowledge” is knowledge independent of the agent claiming the knowledge. So the proposition that “2+2=4” is genuinely knowable if and only if its truth is independent of the person proposing it and of the environment in which it is proposed.

For an example of “non-genuine knowledge”, consider the proposition, “Life is worth living”. For nearly all people that is true, for otherwise we would have quite a bit more suicides. But we can not exclude the possibility that people are deluded into complacent acceptance of the status quo; perhaps had they known better, they would indeed have gone on to kill themselves.

Admittedly, epistemology is a vast area in its own right… questions of whether knowledge is necessarily contingent upon experience or not, questions about how our senses mediate our interaction with the “outside world”, whether an “outside world” even exists independently of perception (George Berkeley famously said no), and on and on. My point here isn’t to attempt to resolve questions unresolved since first posed by the pre-Socratics, but to assert that the question of “purpose” is intimately related to the question of knowledge. We can not identify a purpose to our lives until and unless we espouse the idea that purpose is knowable.

It doesn't have to, but we need some meaning and purpose to feel complete. The other animals do more often what nature/genes/hormones urges them to do: do the mating dance, build a nest, have young. We think more, but in the end most of us have young or find something to replace them.